This invention relates to improvements in laser printers of the type which print alphanumeric characters, forms, logos, etc. by rapidly deflecting a laser beam to produce a scan pattern on a photosensitive surface.
In U.S. Pat. No. 4,044,363 issued to G. T. Morgan, there is disclosed a photocomposition system in which an intensity-modulated continuous-wave laser is deflected in mutually perpendicular directions to print lines of alphanumeric characters on a photosensitive surface. In the Morgan system, an entire line of characters is printed during each horizontal scan of the laser beam. This result is achieved by rapidly deflecting the beam upwardly and downwardly while steadily deflecting the beam, at a relatively slow rate, horizontally. During each vertical deflection of the laser beam, one column of picture elements, commonly referred to as "pixels," of the character is printed. After several columns of pixels have been formed in this manner, the entire character is printed. A programmed computer is used to control blanking of the beam as it moves vertically in order to print the desired characters.
In the Morgan system, the rapid vertical movements of the laser beam is accomplished acoustooptically by passing the laser beam through a conventional acoustooptic cell while simultaneously varying the frequency of the acoustic wave travelling therein. Since the angle at which the acoustooptic cell diffracts an incoming beam of radiation is dependent upon the instantaneous frequency of the acoustic wave travelling in the cell, the direction in which the diffracted beam emerges from the cell can be precisely controlled by precisely controlling the acoustic frequency. Control of the acoustic frequency is effected by driving the cell's transducer with a voltage-controlled oscillator (VCO) and using a signal generator to periodically sweep the output frequency of the VCO through a predetermined frequency range. As the frequency of the VCO output increases, the angle at which the cell diffracts the laser beam increases.
In the Morgan system, as in all laser printing systems in which a beam of radiation is acoustooptically deflected to produce a raster scan, the maximum printing speed is limited by the rate at which the acoustooptic device is capable of deflecting the beam through the requisite scan angle. In systems of the type described above, this rate is determined by the tuning rate of the voltage-controlled oscillator which, in turn, depends upon the frequency response of its input circuits. The maximum linear tuning rate of conventional voltage-controlled oscillators is about 5 MHz per microsecond. This means that the maximum printing speed of such systems is limited to approximately 1000 lines per minute, or approximately 1500 characters per second.